Wireline and logging while drilling (LWD) tools are often used to measure physical properties of the formations through which a borehole traverses. Such logging techniques include, for example, natural gamma ray, spectral density, neutron density, inductive and galvanic resistivity, acoustic velocity, acoustic calliper, downhole pressure, and the like. Formations having recoverable hydrocarbons typically include certain well-known physical properties, for example, resistivity, porosity (density), and acoustic velocity values in a certain range. In many applications (particularly LWD applications) it is desirable to make azimuthally sensitive measurements of the formation properties and in particular, images derived from such azimuthally sensitive measurements, which may be utilized, for example, to locate faults and dips that may occur in the various layers that make up the strata.
Prior art borehole imaging techniques utilize a measured tool azimuth to register azimuthally sensitive sensor data and assume that the measured tool azimuth is substantially identical to the true borehole azimuth. Such techniques are generally suitable for wireline applications in which the logging tool is typically centered in the borehole and thus in which the tool and borehole azimuths are typically substantially identical. However, in LWD applications, an LWD tool is not typically centered in the borehole (i.e., the longitudinal axes of the tool and the borehole are not coincident) since the tool is coupled to a drill string. It is well known that a drill string is often substantially free to translate laterally in the borehole (e.g., during drilling) such that the eccentricity of an LWD tool in the borehole may change with time. Therefore, the assumption that tool and borehole azimuths are substantially identical is not typically valid for LWD applications. Rather, such an assumption often leads to misregistration of LWD sensor data and may therefore result image distortion.
It will therefore be appreciated that there exists a need for improved LWD borehole imaging techniques. In particular, a need exists for a method of determining borehole azimuths. Such borehole azimuths may then be utilized, for example, to register azimuthally sensitive LWD sensor data and thereby form improved borehole images.